1. Field of the Invention
The present invention relates to the redispersion of metals on supported zeolite-containing hydrocarbon conversion catalysts. More particularly, this invention relates to a process for the redispersion of noble metals on supported zeolite-containing hydrocarbon conversion catalysts on which said metals have become maldistributed due to exposure to excessive heat and/or steam.
2. Description of the Prior Art
Crystalline aluminosilicate zeolites, which are commonly referred to as "molecular sieves", are now well known in the art. They are characterized by their highly ordered crystalline structure and uniformly dimensioned pores, and are distinguishable from each other on the basis of composition, crystalline structure, adsorption properties and the like. The term "molecular sieves" is derived from the ability of these zeolite materials to selectively adsorb molecules on the basis of their size and form. Various types of molecular sieves may be classified according to the size of the molecules which will be rejected (i.e. not adsorbed) by a particular sieve. A number of these zeolite materials are described, for example, in U.S. Pat. Nos. 3,013,982-86, wherein they are characterized by their compositions and X-ray diffraction characteristics. In addition to their extensive use as adsorbents for hydrocarbon separation processes and the like, it has been found that crystalline aluminosilicate zeolites, particularly after cation exchange to reduce their initial alkali metal oxide content, are valuable catalytic materials for various processes, particularly hydrocarbon conversion processes. The ion exchange of these crystalline aluminosilicate zeolites with various metals and metal ions is described, for example, in U.S. Pat. Nos. 2,971,904 and Re. 26,188.
In general, the chemical formula of the anhydrous form of the crystalline aluminosilicate zeolite, expressed in terms of mole ratios of oxides, may be represented as: ##EQU1## WHEREIN "M" is selected from the group consisting of hydrogen, monvalent, divalent and tribalent metal cations and mixtures thereof; "n" is its valence, and "X" is a number from about 1.5 to about 12, said value dependent upon the particular type of zeolite. The zeolite, as synthetically produced or found naturally, normally contains an alkali metal such as sodium or an alkaline earth metal such as calcium. Among the well-known naturally occurring zeolites are mordenite, faujasite, chabazite, gmelinite, analcine, erionite, etc. Such zeolites differ in structure, composition, and particularly in the ratio of silica to alumina contained in the crystal lattice structure. Similarly, the various types of synthetic crystalline zeolites, e.g. synthetic faujasite, mordenite, erionite, etc. will also have varying silica-to-alumina ratios depending upon such variables as the composition of the crystallization mixture reaction conditions, etc. Silica-to-alumina ratios higher than 12, e.g., as high as 90 to 100, can also be achieved through various methods for the removal of alumina from the crystal structure of the zeolites. Such zeolites having these higher silica-to-alumina mole ratios are preferred in hydrocarbon conversion processes because of their high stability at elevated temperatures. Those silica-to-alumina ratios above three are particularly preferred.
For use in many commerical operations, e.g. moving or fluidized bed, adsorption or hydrocarbon conversion processes, the difficulty of handling the extremely fine size zeolite crystals has required the use of various "matrices" in conjunction with the zeolites. The use of such matrices is described in U.S. Pat. Nos. 3,140,249 and 3,140,253. The matrix may include various organic and/or inorganic compositions, particularly inorganic oxide gel compositions, including plural gel compositions containing a major amount of silica in conjunction with one or more metal oxides selected from Groups I-B through VIII of the Periodic Table, particularly such compounds as alumina, magnesia, zirconia, titania, etc. The matrix may also include such compositions as clay materials, particularly kaolin-type clays.
Recently such crystalline aluminosilicate zeolite-containing hydrocarbon conversion catalysts of greatly improved stability have been discovered which contain reduced amounts of alkali metal, such as less than 3 wt. %, and preferably less than 1 wt. %, e.g., preferably in the range of between 0.01 to 1 wt. %, more preferably between 0.05 to 1 wt. % (see U.S. Pat. No. 3,449,070).
In the case of dual-function catalysts (such as those used in hydrocracking and isomerization processes), it is desirable to maintain an active hydrogenating site wherein the metals are well dispersed closely adjacent to an acid cracking site in the zeolitic structure.
Typically, such crystalline aluminosilicate zeolite-containing catalysts are utilized in hydrocarbon conversion reactions at various temperatures, generally ranging from about 200.degree. to 1000.degree. F., depending on the particular reaction desired, and utilizing various hydrocarbon-containing feedstreams, such as light virgin naphtha, heavy gas oil, catalytic cycle stock, heavy reformate, coker gas oil, etc. In general, these reactions are conducted at pressures of from about atmospheric to 2200 psig. During such reactions, the zeolite-containing catalysts tend to become deactivated, and are generally cyclically regenerated by high-temperature treatment with an oxygen-containing gas in order to burn-off accumulated carbon and to restore some of their lost activity.
It is well known that the maldistribution of the metals on a supported zeolite-containing hydrocarbon conversion catalyst may result from long term exposure to process conditions which may be aggravated by overheating or contact with excessive partial pressures of water vapor at high temperatures, such may also occur during the oxidative regeneration of the catalyst mentioned above. Under particularly severe conditions, this may result in agglomeration of the metal into large crystallites, e.g. crystallites ranging in size from about 100-200 A or more in diameter.
Various techniques have been suggested to obtain metals redispersion on a hydrocarbon conversion catalyst. One involves contacting a catalyst containing a Group VIII non-noble metal or compound with an aqueous acid solution (see U.S. Pat. Nos. 3,235,486 and 3,256,205). However, this technique, although useful for redispersing non-noble metals on an amorphous support, is not, in general, suitable for redispersing noble metals on zeolite-containing catalysts in that the acid and conditions employed to obtain solubility of the metal are so severe as to cause substantial damage to the crystalline portion of the zeolite. Another method involves rejuvenating siliceous zeolite catalysts comprising zeolitic mono- and/or divalent metal cations and a Group VIII metal hydrogenating component supported thereon by a sequential treatment with an aqueous ammonium salt to exchange out at least a portion of the zeolitic mono- and/or divalent metal ions, and with aqueous ammonia to effect a redistribution of the Group VIII metal (see U.S. Pat. Nos. 3,835,028 and 3,899,441).
However, none of the prior art references suggests redispersing noble metals on a supported zeolite-containing hydrocarbon conversion catalyst using EDTA and/or its salts.